This invention relates in general to the forming of structures from fiber reinforced thermoplastic resins and, more specifically, to such forming using controlled elastomeric tooling.
Recently, light weight, high strength structures fabricated from fiber reinforced thermoplastic resin composites have come into greater use, especially in aircraft, space vehicles and other similar systems. Composites have generally been limited to high value systems and simple structures because of very high fabrication costs.
The composites from which structures are fabricated are mainly available as a fiber fabric or matt and a liquid resin which are to be combined during product fabrication, or in "preforms" in which the fiber fabric or matt is impregnated with the resin in a tacky stage. The resin-impregnated fabric must be laid up in a mold and heated while subjected to uniform pressure to obtain high density in the thermally cured resin. Uniform pressure application has been difficult to obtain in complex mold assemblies, requiring careful mold assembly and adjustment by highly skilled workers.
Attempts have been made to use elastomers having positive coefficients of thermal expansion as the pressure application means. The composite is laid up on rigid mold surfaces, elastomeric blocks are placed against the exposed composite surface and locked in place. Careful design of molds and selection of elastomeric composition and configuration has often provided acceptable results. However, a number of problems remain with this system. As the elastomer ages and goes through a series of molding cycles, its physical characteristics change and the material tends to shrink. This requires regular replacement of the elastomeric components, and continued adjustment and shimming of the mold through succeeding molding cycles.
Also, with the elastomer firmly in contact with the composite, expansion of the elastomer begins as heating begins, so that pressure is immediately applied to the composite, preventing the necessary venting of the volatile agents which the composite outgasses during initial heating. This tends to produce bubbles and weak areas in the final structure. On the other hand, delay in applying pressure beyond the temperature at which curing of the thermosetting resins begins results in inadequate consolidation of the structural member and lower strength.
Thus, there is a continuing need for methods and apparatus for molding and curing structures comprising composite materials.